A conventional excavating apparatus for excavating a ground to form a continuous trench below a ground surface comprises a lower traveling body equipped with a crawler for traveling on the ground, an upper slewing body mounted on the under traveling body, and a portal frame provided in the upper slewing body. This portal frame is provided with a pair of traverse cylinders arranged one-above-the-other, and a leader. The pair of traverse cylinders are operable to slidingly move the leader in a traverse direction parallel to the ground surface. The excavating apparatus further comprises a cutter post and a chain-type cutter. The cutter post is suspended from the leader, and the chain-type cutter is configured to be circulatingly moved in an upward-downward direction while being guided by the cutter post. The chain-type cutter comprises an endless chain configured to be circulatingly driven, and a plurality of bit plates arranged on the endless chain on the side of an outer periphery thereof, at intervals along a circulating movement direction of the endless chain. On each of the bit plates, a plurality of excavation bits is arranged. By moving the cutter post below the ground surface in the traverse direction while circulatingly moving the chain-type cutters, a trench is excavated in a forward movement direction of the cutter post.
JP 2007-56664A discloses an excavating apparatus comprising a flat-shaped T-slotted plate holder attached to bit plates (excavating/agitating vanes) and a base plate attached to a endless chain. In this excavating apparatus, the plate holders and the base plates are fitted together, and, in this state, the bit plates and the endless chain are fixed together by a bolt and a nut.
In the technique described in JP 2007-56664A, the plate holder and the base plate lie in a connection region between each of the bit plates and the endless chain, so that the weight of a chain-type cutter is increased. Thus, a load to be imposed on the chain during circulating movement becomes larger, leading to a problem that damage of the chain is likely to occur. Further, due to variations in shape tolerance of the plate holder and the base plate, or depending on a fastening state of the bolt disposed across the plate holder and the base plate, a relative position of a plurality of excavation bits is likely to change, leading to a problem that an excavation plane becomes unstable. Moreover, the set of the plate holder and the base plate lie between the bolt and the nut, so that, due to reaction forces applied to the bit plate during excavation, a strong shear force is applied to the bolt, leading to a problem that loosening or disengagement of the bolt occurs.